librbd: deep-copy object copy should register an in-flight op

librbd: deep-copy object copy should register an in-flight op

When handling live migrations, the source image is the parent
image of the destination image. To prevent the parent image from
being closed while a request is in-flight, the object copy
state machine now registers an async operation with the source
image.

Signed-off-by: Jason Dillaman <dillaman@redhat.com>
(cherry picked from commit 89cfa6c8d328b9294c7f7efd53d82edcec59e66d)